[PATCH] PCI: DMA bursting advice

After seeing, at best, "guesses" as to the following kind
of information in several drivers, I decided that we really
need a way for platforms to specifically give advice in this
area for what works best with their PCI controller implementation.

Basically, this new interface gives DMA bursting advice on
PCI.  There are three forms of the advice:

1) Burst as much as possible, it is not necessary to end bursts
   on some particular boundary for best performance.

2) Burst on some byte count multiple.  A DMA burst to some multiple of
   number of bytes may be done, but it is important to end the burst
   on an exact multiple for best performance.

   The best example of this I am aware of are the PPC64 PCI
   controllers, where if you end a burst mid-cacheline then
   chip has to refetch the data and the IOMMU translations
   which hurts performance a lot.

3) Burst on a single byte count multiple.  Bursts shall end
   exactly on the next multiple boundary for best performance.

   Sparc64 and Alpha's PCI controllers operate this way.  They
   disconnect any device which tries to burst across a cacheline
   boundary.

   Actually, newer sparc64 PCI controllers do not have this behavior.
   That is why the "pdev" is passed into the interface, so I can
   add code later to check which PCI controller the system is using
   and give advice accordingly.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 17 insertions, 0 deletions

diff --git a/include/asm-ppc64/pci.h b/include/asm-ppc64/pci.h
index 411bf5dee394..20beb10c0902 100644
--- a/include/asm-ppc64/pci.h
+++ b/include/asm-ppc64/pci.h
@@ -78,6 +78,23 @@ static inline int pci_dac_dma_supported(struct pci_dev *hwdev,u64 mask)
         return 0;
 }
 
+static inline void pci_dma_burst_advice(struct pci_dev *pdev,
+                                        enum pci_dma_burst_strategy *strat,
+                                        unsigned long *strategy_parameter)
+{
+        unsigned long cacheline_size;
+        u8 byte;
+
+        pci_read_config_byte(pdev, PCI_CACHE_LINE_SIZE, &byte);
+        if (byte == 0)
+                cacheline_size = 1024;
+        else
+                cacheline_size = (int) byte * 4;
+
+        *strat = PCI_DMA_BURST_MULTIPLE;
+        *strategy_parameter = cacheline_size;
+}
+
 extern int pci_domain_nr(struct pci_bus *bus);
 
 /* Decide whether to display the domain number in /proc */